It is of utmost importance to identify the circuits underlying learning and memory in order to understand not only the mechanisms of memory but also how these mechanisms become dysregulated during age-related cognitive decline (ARCD) and Alzheimer's disease (AD). Here, we will characterize the neural circuitry underlying brain plasticity and resilience as it occurs during cognitive loss in ARCD and AD with single-cell resolution. In this proposal, the individual neurons corresponding to an individual memory will be identified by using an activity-dependent transgenic line, the ArcCreERT2 mice. This mouse line allows for the indelible labeling of cells expressing the immediate early gene (IEG) Arc/Arg3.1 and allows for a comparison between the cells that are activated during the encoding of a memory and those that are activated during the retrieval of the corresponding memory. In combination with optogenetic reporter lines and/or viral injection strategies, these studies will assess the involvement of individual neurons in memory encoding and retrieval in naturally aged or AD (APP/PS1 or 3xTg-AD) ArcCreERT2 mice. To visualize and manipulate these neural ensembles, and thus, the corresponding circuits in memory, we will selectively express the blue light activated cation channel channelrhodopsin-2 (ChR2) in Arc+ cells activated during memory encoding. Whole-brain memories will first be visualized in order to determine which neural ensembles become dysregulated following aging and AD development. Identification of similarities and differences and thus, susceptibility and resiliency, between the ensembles will be performed using neuronal modeling developed in the Denny laboratory. In Aim 2, we will use in vivo Ca2+ imaging to better understand the dynamics (e.g., Ca2+ activity) of neural ensembles as they participate in memory encoding and retrieval in aged and AD mice. In Aim 3, the dysregulated neural ensembles in aged and AD mice will be restored using optogenetic modulation. In addition, in vivo Ca2+ imaging will occur during this stimulation in order to determine the mechanism by which modulation rescues cognitive decline. Comprehensive immunohistochemistry, network modeling, Ca2+ imaging, and optogenetic techniques will be utilized. As most studies have narrowed their analyses to a single brain structure, these studies will expand this scope exponentially by analyzing whole-brain memory traces mediating cognitive loss in AD and aging. This proposed study of whole brain memory traces will be conducted in a manner aligned with human imaging studies, and thus, is likely to provide novel and translational insights into the neural networks mediating memory loss in aging and AD.